The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device, more particularly to a semiconductor device and a method of manufacturing semiconductor devices comprising a bipolar transistor and a field-effect transistor respectively.
These years, semiconductor manufacturers have developed processes for manufacturing higher performance semiconductors. For example, an epitaxial base process which enables shallow junction and low resistance for semiconductor device bases has been developed for manufacturing of bipolar transistors, which is known as one of those semiconductor types. Those bases are very important for manufacturing high performance semiconductor devices. This epitaxial process can form a thin base layer of about 50 nm in high density and high accuracy.
The epitaxial process uses a silicon germanium (SiGe) mixed crystal as a base material having a band gap smaller than that of silicon for a practical heterojunction structure. In the case of the heterojunction, the injection efficiency of carriers into a base from an emitter can be improved more than the homojunction. Thus, a current amplification factor h.sub.FE can be secured without increasing the base resistance and the carrier transition time between emitter and base, resulting in high performance bipolar transistors.
On the other hand, low resistance of a base lead-out electrode is also an important item for high performance bipolar transistors. Conventionally, it is considered to be effective to form part of the base lead-out electrode with high-melting metal whose resistance is lower than that of polysilicon for making the resistance of a base lead-out electrode low.
In the case of manufacturing of bipolar transistors using the epitaxial base process, for example, a base leadout electrode is formed on a substrate, then a base layer comprising an epitaxial layer is formed on the substrate so as to come in contact with the base lead-out electrode.
When manufacturing such a bipolar transistor, however, a heat treatment of 600 to 800.degree. C. is needed for growing an epitaxial layer, which becomes a base layer. So, when a base lead-out electrode is formed with high-melting metal, the high-melting metal in the base lead-out electrode is captured by the epitaxial layer during the epitaxial growth, causing a problem that the epitaxial layer is contaminated. And, when the base layer comprising the epitaxial layer is contaminated, the life time of the carriers contained in the base goes down, causing the deterioration of the electrical property that the base current is increased and the current amplification factor h.sub.FE goes down.